1. Field of the Invention
The present invention relates to a sheet creaser, a sheet finisher including the sheet creaser, an image forming apparatus including the sheet finisher, a sheet folding method, and a computer program product.
2. Description of the Related Art
Image-forming-apparatus connectable bookbinding machines that bind a set of sheets (hereinafter, “sheet set”) by simple saddle stitch have been widely used. There are various needs in the bookbinding machine market such as a bookbinding machine capable of binding more sheets, a bookbinding machine capable of binding thicker sheets, and a bookbinding machine having a cutting function. To fulfill these needs, it is necessary to tightly fold the sheet set. In other words, it is necessary to make the crease stronger.
Re-pressing is a technique to make the crease stronger. The re-pressing means that pressing a folded side of the sheet set twice or more. There are two approaches in the re-pressing. The first approach is to press the folded side twice in the same direction. The second approach is to press the folded side twice in different directions (directions perpendicular to each other). In the first approach, a pair of folding rollers half-folds the sheet set with high pressure while rolling in one direction (positive direction). After that, the folding rollers re-press the folded sheet set while rolling in a reverse direction (negative direction). In the second approach, after the sheet set is passed through a nip between the folding rollers, a pressure roller re-presses the folded sheet set while rolling on the crease.
The second approach has better re-pressing performance, and therefore most of bookbinding systems emphasizing on productivity employ the second approach. In most of the bookbinding systems using the second approach, from the viewpoint of space saving, the pressure roller is arranged near the folding rollers to re-press the sheet set immediately after the folding rollers make the crease. After the pressure roller re-presses the sheet set, the folding rollers convey the sheet set to a tray out of the bookbinding system. A technology disclosed in Japanese Patent Application Laid-open No. 2005-162345 is an example of the second approach.
A sheet finisher disclosed in Japanese Patent Application Laid-open No. 2005-162345 receives the sheets on which images are formed and performs a finishing process on those sheets. The sheet finisher includes a guiding unit, a re-pressing unit, and a supporting unit, as salient features. The guiding unit guides, after the sheet set is aligned and half-folded, the folded sheet set, carrying the folded sheet set on a surface of the guiding unit. The re-pressing unit re-presses the folded side of the sheet set, moving in a direction perpendicular to a sheet conveying direction in which the guiding unit conveys the sheet set. The supporting member supports sides of the sheet set while the re-pressing unit is re-pressing the sheet set.
However, in some cases, especially when there are many sheets to be processed in one operation, the conventional sheet finisher cannot make the crease strong enough. It is considered that a manner of conveying the sheet set by the folding rollers affects the strength of the crease. The sheet set is tightly folded immediately after the folding rollers folds the sheets set. However, if the manner of conveying is poor, as shown in FIG. 28A, an inner surface of the folded sheet set gets wavy and the wavy inner surface causes the outer surface to expand. Therefore, a crease SH1 of the sheet set is weak. Even if the pressure roller re-presses the crease SH1 shown in FIG. 28A, because the crease SH1 is swollen, the crease SH1 cannot be strong enough.
A creaser disclosed in Japanese Patent No. 3990256 includes the folding rollers that fold the sheet or the sheet set passing through a nip between the folding rollers, a pressing unit that applies a pressure to the folding rollers when the folding rollers fold the sheet or the sheet set, a pressure changing unit that changes the applied pressure depending on a conveying state of the sheet or the sheet set. Components of the pressing unit are arranged substantially symmetrically with respect to the center of a conveyer path, through which the sheet or the sheet set passes, running through the nip between the folding rollers. The pressing unit includes a first elastic member that generates a first biasing force, a first transmission member that transmits the first biasing force to the folding rollers, a second elastic member that generates a second biasing force, a second transmission member that transmits the second biasing force to the folding rollers. The first biasing force is set smaller than the second biasing force. The pressure changing unit changes the pressure by switching between the first biasing force and the second biasing force.
In the creaser disclosed in Japanese Patent No. 3990256, when a leading end of the sheet enters the nip between the folding rollers, the pressure changing unit causes the first transmission member to transmit the first biasing force to the folding rollers. When the leading end passes the nip, the pressure changing unit causes the second transmission member to transmit the second biasing force to the folding rollers.
However, even in the creaser disclosed in Japanese Patent No. 3990256, the state when the re-pressing roller re-presses the sheet set is unchanged, i.e., the sheet is in the state as shown in FIG. 28A. Therefore, as described above, the creaser cannot make the crease strong enough.